The production of polyamides, including polytetramethylene adipamide, is extensively described in U.S. Pat. Nos. 2,130,478 and 2,163,584, which also disclose the general principle for obtaining product of high molecular weight. Of the general requirements that are of particular importance for the production of high-molecular-weight polytetramethylene adipamide the following are mentioned:
a. As starting material use is made of virtually equimolar amounts of monomer, preferably in the form of the salt thereof. PA1 b. The polycondensation can be effected in the presence or absence of a diluent. PA1 c. To effect a shift in the balance of the polycondensation reaction to high-molecular-weight polyamide it is necessary to discharge water that is formed for example as a reaction product at the end of the polymerisation at least. PA1 d. If one of the monomers is volatile, this monomer has to be supplied in an excess amount, of about 5% at most, at the start or has to be recirculated, optionally after fractionated distillation, or the polycondensation has to be carried out under pressure in a sealed reactor, in which case water vapour is discharged at atmospheric or lowered pressure only in a last process step.
The principle disclosed by Carothers has been implemented for the preparation of high-molecular-weight polytetramethylene adipamide in for example the following patents: JP-A-47-41111: Polycondensation of equimolar amounts of monomer in an autoclave under elevated pressure and subsequently for some time with release of the pressure, EP-B-0077106, EP-B-0038094 and EP-B-0160337: Polycondensation with an excess of 1,4-diaminobutane in an autoclave under elevated pressure followed by aftercondensation in the solid phase at atmospheric pressure and EP-A-0393544: Using equimolar amounts of the monomers or the salt at a slightly elevated pressure (1-10 bar) with recirculation to the beginning of the process of the 1,4-diaminobutane which is discharged, along with other volatile components, during the polycondensation via the vapour phase and is separated therefrom. The polycondensate obtained is then to be condensed further in the solid phase to obtain a high-molecular-weight product. In this last process, too, an excess of 1,4-diaminobutane is in fact supplied at the beginning of the polycondensation process.
However, the various disclosed processes present the following drawbacks.
With the process of JP-A-47-41111 it appears to be virtually impossible to obtain high-molecular-weight polyamide: .eta..sub.rel max. is approx. 2.2. The process described in the three EP-B publications present the drawback that the first process step, under elevated pressure, requires a relatively large amount of time, namely several hours. The process disclosed in EP-A-0393544 on the other hand presents the advantage of a particularly short first process step, but the polycondensate obtained with said process has only a relatively low degree of polymerisation, namely corresponding to a .eta..sub.rel of 1.5, and requires a particularly long aftercondensation of 10 to 40 hours in the solid phase. Because of the low degree of polymerisation of the polycondensate obtained after the first process step it is virtually impossible to draw fibres from it, as a result of which a very irregular prepolymer granulate is obtained, which presents practical drawbacks for the solid-phase aftercondensation and further processing.